According to the National Center for Health Statistics, cardiovascular disease is the leading cause of death in the United States. Carotid atherosclerosis is one of the main causes of stroke. Atherosclerosis is a form of arteriosclerosis characterized by the deposition of atheromatous plaques containing cholesterol and lipids on the innermost layer of the walls of large- and medium-sized arteries. Improved methods of diagnosis, treatment, and prevention of these diseases would result in a significant improvement in the quality of life of patients and a concomitant decrease in health care costs.
Traditionally, the degree to which lumen stenosis has occurred is used as a morphological marker for high risk (i.e., vulnerable) plaques. Clinical, X-ray, computerized tomography (CT), ultrasound, and magnetic resonance (MR) angiography are used to determine lumen stenosis. The determination of lumen stenosis lacks a unified approach, as evidenced by the two different methods of stenosis quantification that are most often used—the North American Symptomatic Carotid Endarterectomy Trial (NASCET) and the European Carotid Surgery Trial. Regardless of the method used, it has been demonstrated that lumen narrowing is a poor indicator of patient vulnerability to strokes. In symptomatic patients, evidence of lumen narrowing predicted only about one out of four strokes. In asymptomatic patients, evidence of lumen narrowing predicted only about one out of ten strokes. Clearly, it would be desirable to employ a more accurate predictor of a patient's risk of having a stroke using conventional imaging procedures.
Some studies indicate the importance of plaque morphological factors (in addition to stenosis) in determining thromboembolic risk. Specifically, ultrasonographic studies show that plaque thickness is a better predictor of transient ischemic attacks than vessel stenosis. Unfortunately, ultrasound measurement of plaque thickness is not highly reproducible, since such measurements produce results varying from 13.8% to 22.4%. Further evidence of the importance of plaque morphology was documented in a study by NASCET investigators showing increasing risk for stroke with ulcerated plaques, as compared to non-ulcerated plaques with similar degrees of stenosis. These results suggest that a comprehensive, quantitative analysis of plaque morphology, including lumen stenosis, wall thickness, and ulceration, will better identify vulnerable plaques.
Recent studies have shown that Magnetic Resonance Imaging (MRI) is capable of identifying plaque constituents and measuring plaque morphology. MR plaque imaging may therefore be a useful technique for characterizing plaque morphology and tissue constituents in one examination, thereby assessing both aspects of vulnerable plaque. However, current MRI techniques fail to provide the data necessary for comprehensive, quantitative analysis of plaque morphology. It would be desirable to develop methods of using MRI to obtain accurate tracing of lumen and wall boundaries, a reasonable definition of carotid wall thickness, accurate computation of lumen surface roughness, and a reasonable definition of plaque burden indexes. Techniques for tracing lumen and wall boundaries and computing lumen surface roughness have been suggested. However, a consistent and reliable technique for estimating lumen wall thickness from an image of a lumen has not been taught in the prior art.
In addition to wall thickness, other quantitative morphological parameters can be defined to assess plaque morphology. Morphological description refers to the methods that produce numeric morphological descriptors and is carried out subsequent to morphological representation. A morphological description method generates a morphological descriptor vector (also called a feature vector) from a given shape. The goal of morphological description is to uniquely characterize a shape using its morphological descriptor vector.
Previous work has established the reproducibility of in-vivo measurements of area and volume from a carotid artery MR image. These studies indicate a good agreement between in-vivo and ex-vivo measurements. Specifically, volume measurements matched to within 4%-6% and cross-sectional area measurements matched to within 5%-11% for two independent MR scans performed within 2 weeks that were reviewed by two independent reviewers. In addition, it has been determined that different contrast weighted images (T1, T2, and proton density) of comparable image quality will yield similar results in lumen and vessel wall area measurements. Such results indicate that morphological descriptors extracted from MRI may be used to characterize vascular shape variance. However, the prior art does not define a specific set of useful morphological descriptors. It would be desirable to provide vascular morphological descriptors that are based on lumen boundary, wall boundary, and wall thickness, and to use such descriptors to evaluate plaque morphology.